Abstract : Laser - Induced Fluorescence (LIF) was incorporated to study the degree of mixing in two subsonic nozzle variations for the O-I laser system. The studies were performed using a testbed version of COIL-IV, an O-I laser. I was injected through the nozzles and excited with an Ar laser to produce fluorescence. The observed jet trajectories could then be compared to trajectories predicted by computer models. Velocity distributions were measured at several flow rates. The LIF allowed for an evaluation of the relative degree of mixing produced by the two nozzles resulting in a modification of the O-I laser. Keywords: Oxygen iodine, Laser induced fluorescence, Nozzle, Fluid dynamics, Subsonic mixing.